Peripheral nerve injuries can be caused by trauma and iatrogenic injury. Over 700,000 peripheral nerve injuries occur due to accidental trauma and during surgery in the US and Europe annually. In the majority of injuries the nerve ends cannot be opposed. There are currently two choices for repair in this situation. Though an autologous nerve graft is the gold standard of nerve graft repair, it has several disadvantages, including the need for an extra incision, loss of donor nerve function, mismatch in size between the donor nerve and the injured nerve, and a limited availability of donor nerve. The second promising alternative is to use a synthetic “nerve tube” as a conduit for regeneration. A variety of synthetic and natural biopolymers such as polyglycolic acid, polylactic acid, chitosan, alginate silk and their composites or derivatives, has shown varying levels of success and • aws [Jiao et al, 2009; Yang et al, 2007; Matsmoto et al, 2000; Evans et al, 2002; Toba et al, 2002; Thomas et al, 2005; Wang et al, 2005; Bellamkonda, 2006; Jiang et al, 2007].
The nerve tissue matrix is composed of basal lamina tubes around the axon-schwann cell unit and longitudinally oriented collagen fibrils. There is extensive and compelling evidence that the success of peripheral nerve regeneration depends on the extra-cellular matrix [Giannini et al, 1990; Sorenson et al, 1993]. Collagen, the main structural component of these structures, is an extracellular matrix (ECM) molecule which may have specific advantages over other synthetic and natural polymers since it possesses cell-adhesive and signalling domains that are critical for nerve regeneration. As collagen may present a highly relevant biological microenvironment for axonal growth, a range of collagen conduits have been developed for nerve regeneration studies [Archibald et al 1991; Collin et al, 1984; Laquerriere et al, 1993].
Previous nerve regeneration studies have been performed dominantly based on single channel conduits [Jiao et al, 2009; Yang et al, 2007; Matsmoto et al, 2000; Evans et al, 2002; Toba et al, 2002; Thomas et al, 2005; Wang et al, 2005; Bellamkonda et al, 2006; Jiang et al, 2007]. Fibrous structure and microgrooves have been investigated and introduced into the design of nerve conduits to improve axonal growth guidance [Chew et al, 2007; Yang et al, 2008; Kim et al, 2007; Yao et al, 2009 1, 2;]. However, regardless of surface microtopography, single-channel nerve tube repair may lead to inappropriate target reinnervation by the dispersion of regenerating axons across the graft. [Brushart et al, 1995; de Ruiter et al, 2008b]. Although a previous nerve repair study based on PLGA multichannel conduits indicated less axonal dispersion compared with single channel conduit, most PLGA conduits swelled and collapsed in the experimental time-frame (de Ruiter G et al, 2008b). Multichannel collagen conduits that resemble the structure of nerve multiple basal lamina tubes may limit the dispersion of regenerating axons and provide guidance for nerve growth. When fabricating the next generation of nerve conduits, several aspects need to be considered. These include the biocompatibility of the materials, and the customized mechanical and degradation properties. Single-channel collagen conduits that possess a mechanical strength and are easy to process have previously been used in nerve regeneration applications. Because the channels of multichannel collagen conduits are much smaller than single channel conduits, slight swelling and deformation may reduce channel cavities significantly and impede nerve growth. The fabrication of multichannel collagen conduits is challenging and needs to be well programmed and optimized.
A number of nerve conduits are FDA approved for relatively short nerve defects, such as Integra Neurosciences Type I collagen tube, NeuraGen™, Collagen Matrix Inc.'s neuroflex and Synovis Surgical Innovations Gem Neurptube™. These are single-channel tubes which are used only for small defects of several millimeters and do not address larger peripheral nerve injuries. In addition, axons regenerating across these single lumen tubes assume a dispersed direction, resulting in inappropriate target re-enervation and the co-contraction of opposing muscles or synkinesis. The advantage of multichannel nerve conduits is that the dispersion may be limited as they resemble the structure of nerve multiple basal lamina tubes.
The goal of this study was to develop a robust collagen-based nerve conduit with multiple dimensionally stable sub-millimeter diameter channels to facilitate nerve guidance and limit dispersion. Towards this goal, we developed an innovative multi-step (sequential) moulding technique to create 1-, 4-, and 7-channel conduits from a high concentration collagen solution. The conduits were crosslinked with (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) in N-hydroxysuccinimide (NHS)) to control the biodegradation rate and to limit swelling.
To examine biocompatibility of the crosslinked collagen, dorsal root ganglia were cultured on the substrates and axonal growth quantified. As mechanical robustness and elastic recoil of the conduits is essential for implantation and post-surgical success, the structural mechanical properties were characterized by tensile, compressive, and three-point bending tests. This combination of chemical, biological, and mechanical analyses is necessary to determine the optimal biomaterial design for implantation studies.
The conduits were implanted in a 1 cm defect of the rat sciatic nerve and nerve regeneration was evaluated using compound muscle action potential (CMAP) recordings, quantitative nerve morphometry and simultaneous retrograde axonal tracing studies.